Preparation of mcr3o8,where m is an alkali metal,from cro3,cr2o3 and m2cr2o7



United States Patent O 3,450,491 PREPARATION OF MCr O WHERE M IS AN ALKALI METAL, FROM CrO Cr O AND Mgcl'zoq Joseph H. Balthis, Jr., Mendenhall, Pa., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Original application Dec. 14, 1965, Ser. No. 513,841. Divided and this application Aug. 1, 1968, Ser. No. 749,294

Int. Cl. Clg 37/02 U.S. C]. 23-56 7 Claims ABSTRACT OF THE DISCLOSURE Described and claimed is the novel process for making the known alkali metal chromates MCr O e.g., KCr O by heating a mixture of CrO Cr O and an alkali metal dichromate at 170-400" C. The compounds of the formula MCr O are useful in the synthesis of ferromagnetic chromium oxide.

RELATED APPLICATION This application is a division of my copending application, Ser. No. 513,841, filed Dec. 14, 1965.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to, and has as its principal object provision of, a novel process for making the known alkali metal chromium oxides containing the Cr O grouping, useful for preparing ferromagnetic chromium oxide.

Description of the prior art DESCRIPTION OF THE INVENTION In accordance with the present invention, it has now been discovered that MCr O can be obtained in unusually finely divided physical form, particularly suitable for conversion to high coercivity CrO by heating a mixture of CrO Cr O and an alkali metal dichromate for O.l12 hours at ITO-400 C. and a pressure ranging from atmospheric or below to 1000 atmospheres or higher. The

3,450,491 Patented June 17, 1969 reaction involved is believed to be represented by the following equation:

When the reaction is conducted at atmospheric pressure or below, use of anhydrous reactants is not essential since any water initially present escapes during early stages of the heating cycle. Under superatmospheric pressure, however, the water content of the reaction mixture is important and in no case should the proportion of water present exceed an amount equivalent to two moles per mole of dichromate.

Anhydrous and dihydrated alkali metal dichromates or mixtures thereof can be used at 270-350 C. (and pressures of 1-1000 atmospheres), but at higher temperatures, for example, 400 C. (1000- atmospheres), substantially anhydrous dichromate is required for production of high quality MCr O For economic reasons, it is preferred to prepare MCr O at atmospheric pressure. The CrO Cr O and the dichromate are preferably initimately mixed before reaction, and may be brought to reaction temperature rapidly. A several-fold excess of dichromate over that required by Equation 2 is normally employed since the presence of water-soluble dichromate facilitates washing the black, water-insoluble MCr O from the reactor.

In preparing MCr O by this novel process, a reactive form of Cr O is used. This may be prepared by precipitation from aqueous chromium( III nitrate or chromium(III) chloride solution using ammonium hydroxide. The precipitated hydrous chromium(III) oxide is thoroughly washed with water to remove nitrate or chloride anions and is then heated or calcined at substantially atmospheric pressure, i.e., at a pressure in the range of 0.5-5 atmospheres and a temperature of 200-1000 C. This heating is preferably carried out under oxidizing conditions, i.e., in the presence of air or oxygen, to yield a product which may in certain cases contain chromium with an average valence above 3 but less than 4. Periods of time ranging from a few minutes, e.g., 10 minutes, to several hours, e.g., 24 hours, are usually sufiicient for the heating step.

Depending upon the reaction conditions selected, the preparation of MCr O may be effected in open vessels or in closed pressure vessels. The preparation is preferably carried out in an atmosphere of air or oxygen though other chemically inert gases may be used. Reaction in flowing oxygen is advantageous.

There follow some examples which illustrate the process of the invention in more detail.

EXAMPLES AL These examples illustrate the preparation of alkali metal chromium oxides, MCr O The reactants employed, the reaction conditions, and the characteristics of the products are described in the table. Examples J. K and L are controls which show the importance of controlling the amount of water used in the claimed process: if 0.5 g. of H 0 is added to the reactant mixture of Example C (Example J), or 0.6 g. to the mixture of Example D (Example K), the product is CrO Similarly, use of Na Cr O -2H O in place of K Cr O of Example D (Example L) results in formation of CrO TABLE-PREPARATION OF MCraOs Reactants (g.) Reaction Conditions Example Temp, Pressure, Time Maximum Particle Size (Microns) No. CrO; OM MzCI207 C. Atm. in Hrs. Product (Length Width) 4 2 NazCrzO aHgO, l5 225 135-233 12 NaCnO (Not resolved under 60-fold magnification). 2 1 350 500 6 NaCrgOg 3 (Non-aeicular, angular particles). 2 1 350 1, 000 6 NaCraOs 7, 4. 2 1 400 1, 000 e KCrsOs 10, 3. 2 1 350 5 6 NaCraOg 11, 5. 4 2 333 1 6 N aCraO 4 2 400 1 6 NaCnOg 4 2 200 1 5.5 NaCrgog 4 2 NazCrzO -2HzO, 10 173-263 1 0.25 NaCr O 2 1 N212CrzO7-2HzO, 5; +0.5 g. H20. 350 1, 000 -6 CrOz 8, 0.2. 2 1 K2GrzO1+0.6 g. H10, 5 400 1, 000 6 CrOa 6, 0.25. 2 1 NazCt207-2H20, 5 400 1, 000 6 C1702 25, 0.5.

1 These preparations were carried out in an atmosphere of flowing oxygen saturated with water at 92 G.

Since obvious modifications and equivalents in the in- 4. The process of claim 1 in which the dichromate is vention will be evident to those skilled in the chemical that of sodium. arts, I propose to be bound solely by the appended 5. The process of claim 1 in which the dichromate is claims. that of potassium.

The embodiments of the invention in which an ex- 6. The process of claim 1 accomplished in the presclusive property or privilege is claimed are defined as ence of oxygen. follows: 7. The process of claim 6 in which the oxygen is 1. The process of preparing an alkali metal chromium flowing. oxide MCr O wherein M is lithium, sodium, potassium, References Cited rubidium or cesium which comprises heating a mixture of CrO a reactive form of Cr O and an alkali metal di- 150 NO 2 May 1963 chromate for at least 0.1 hour at a temperature in the silchow aL i of American chemical range 170400 C., any water present being in amount ciety VOL 74 1952 1678 1679 never more than two moles per mole of dichromate and, wilhelmi A Chemica scandinavica VOL 12 1958 at above 350 0, always being less than two moles per 19654576- mole of dichromate.

2. The process of claim 1 in which the ingredients are substantially anhydrous.

3. The process of claim 1 in which an excess of di- 35 US Cl XR chromate over that stoichiometrically required is employed. 23145 Slinkin et al., Dorlady Akademii Nauk SSSR, vol.

H. T. CARTER, Primary Examiner. 

